Ethylene polymers of both the low density type and the high density type are used in large volume to manufacture blow molded articles such as containers. In the manufacture of such articles, the ethylene polymer is extruded in the form of a tube, which frequently is referred to as a parison. The freshly extruded parison is blown to form the desired article. Such processes are quite efficient in the fabrication of modest size containers weighing up to about 100 grams and having volumetric capacities of up to about 1 gallon. Difficulties are encountered, however, in preparing larger containers of desired strength at high production rates. Difficulties also are encountered in preparing both small and large containers from ethylene polymers having very high molecular weights or low melt indexes.
By reason of the relatively low cost of ethylene polymers, there is a growing interest in employing such polymers to prepare larger articles having volumeteric capacities of up to about 50 gallons or even larger. For a number of reasons, difficulties are encountered in preparing such large articles. Specifically the parisons required to prepare such large blow molded articles are longer in length than the parisons employed to prepare smaller articles and are significantly heavier per unit length. The molten parisons have relatively little strength at the temperature at which they are extruded and large heavy parisons tend to draw down excessively as the molten polymer leaves the extrusion die. This leads to numerous problems including the maintenance of proper wall thickness in the parison and the blow molded article prepared therefrom.
It is known that the melt strength of the extruded parison is a function of the viscosity of the molten ethylene polymer contained therein at its prevailing temperature. It also is known that the viscosity of any molten ethylene polymer is a function of its temperature, with the viscosity increasing as the molten ethylene polymer's temperature is lowered. Knowledge of this relationship suggests that the strength of an extruded parison can be increased in either of two ways. When dealing with a specified ethylene polymer, the viscosity of the molten ethylene polymer can be increased by extruding the parison at lower temperatures. Once the extrusion temperature is fixed, the strength of the extruded parison can be increased by selection of an ethylene polymer of higher molecular weight which has a higher viscosity at the designated temperature.
To lower the temperature of the extrudate, it is necessary to reduce the speed of the extruder screw. This lowers the extruder output and increases the operating costs of the process.
The option of improving the melt strength of the parison by employing an ethylene polymer having an inherently higher melt viscosity at the selected extrusion temperature is more attractive. The use of ethylene polymers having higher melt viscosities to prepare blow molded articles is somewhat limited, however, by reason of the fact the the extrusion of such ethylene polymers into parisons sometimes is accompanied by the undesireable phenomenon known in the art as melt fracture. Melt fracture occurs when the shear-stress on the molten polymer exceeds approximately 3-6.times.10.sup.6 dynes/cm.sup.2. Shear stress is the product obtained by multiplying the polymer's apparent melt viscosity in poises by the shear rate in sec.sup.-1.
When an ethylene polymer is extruded under conditions giving rise to melt fracture, the surface of the extrudate leaving the die has an irregular rough surface. Workers in the art sometimes refer to such surfaces as having a "shark skin" appearance. As a minimum, the surface of the extrudate is aesthetically unpleasing. In some cases, the irregular exterior surface has an adverse effect on the physical properties of the ultimate finished article. Thus, it is undesirable to extrude parisons under conditions of melt fracture.
For the reasons noted above, there is a need in the art for an improved process for extruding high molecular weight ethylene polymers into heavy tubular shaped members such as parisons at high productivity rates under conditions which do not give rise to melt fracture.